1. Field of the Invention
This invention relates to an electronic structure, and associated method of fabrication, for reducing corner cracking in flip chip underfill.
2. Description of Background
It is known to form a microelectronic assembly by a direct chip attach method, commonly referred to as flip chip. An example of such microelectronic assembly is the assembly shown in prior art FIGS. 1A and 1B, wherein an integrated circuit die 10 is mounted directly onto a supporting substrate 22. The supporting substrate 22 may be a printed circuit board or a package board with solder bump interconnections 12. The integrated circuit die 10 is spaced apart from the supporting substrate by a gap (not shown), and the solder bump interconnections 12 extend across the gap and connect bond pads 18 on the integrated circuit die 10 to bond pads 20 on the supporting substrate 22. In this manner, the integrated circuit die 10 is attached to the supporting substrate, and electrical signals are conducted to and from the die 10 for processing.
Because of differences in the coefficients of thermal expansion of the die 10 and the supporting substrate 22, stresses are created when the assembly is subjected to thermal cycling of the type experienced during operation. These stresses tend to fatigue the solder bump interconnections 12 and can lead to failure of the assembly. Failure of the assembly manifests itself in the form of cracks, interconnection failure, and the like. In order to strengthen the solder joints without affecting the electrical connection, the gap is filled with a polymeric underfill material 14.
The underfill material 14 is typically applied after the die 10 is attached by the solder bump interconnections 12 to the supporting substrate 22. A curable polymeric underfill material is dispensed onto the supporting substrate 22 adjacent to the die 10 and is drawn into the gap by capillary action. An additional amount of the underfill material is applied along the edges of the die 10 so as to form a uniform fillet 16 that extends beyond the edge of the die 10, and at least partially up the side of the die 10. The polymeric material is then cured, typically by heating, to form the underfill. The underfill bonds to the die 10, the supporting substrate, and the solder bumps, thereby strengthening the assembly and protecting the solder bump interconnections from environmental damage.
The curing process, however, creates thermal stresses during the heating and cooling of the assembly. Mechanical stresses are also experienced during use of the microelectronic assembly, particularly in portable applications such as cell phones, PDA's, and the like. These stresses are normally detrimental to the die and the solder bump interconnections and can cause a detrimental warping of the integrated circuit die. The stresses also lead to cracking of the underfill. As shown in exemplary prior art FIG. 2, a fillet 16 is disposed outside the outer perimeter of the die, i.e., outside the die shadow. The cracking 30 of the underfill commences from the fillet 16 and a corner 32 of the die and propagates, with a force concentrated at the tip of the crack, through the solder bumps which are bonded with the underfill.
Therefore, a need exists to reduce the detrimental effects of thermally induced stresses upon an underfill and the solder bump interconnections of a microelectronic assembly. Further, a need exists to strengthen and improve the reliability of a microelectronic assembly that includes reducing cracks in the underfill to improve the reliability of electrical interconnections and to extend the useful life of an underfilled flip-chip arrangement.